The present invention generally relates to co-electrodeposition of platinum and a supplementary constituent to platinum aluminide coatings, and to methods for forming such coatings.
In modern gas turbine engines, the blades and vanes in the high pressure turbine section are exposed to temperatures in excess of 1000° C. for extended periods of time. Superalloy gas turbine engine components are commonly coated with platinum aluminide coatings to inhibit oxidation and corrosion of the superalloy surface. Protection provided by platinum aluminide coatings is due to selective oxidation of aluminum to form an alumina (Al2O3) scale that grows very slowly at high temperature by a diffusion process.
Impurities within the platinum aluminide coating, e.g., sulfur (S), phosphorus (P), and chlorine (Cl), can segregate to the interface between the coating and the alumina scale, weaken the interface, and thus promote spalling of the protective oxide scale. Periodic oxide spalling accelerates the consumption of aluminum from the platinum aluminide coating and reduces the oxidation life of the component. Impurity-induced oxide spalling of the protective oxide scale also limits the life of thermal barrier coatings that utilize platinum aluminide coatings as a bond coating.
U.S. Pat. No. 6,306,277 to Strangman et al. discloses an electroplating process for electrodeposition of platinum on superalloy substrates, and a platinum electrolyte for use in such electroplating process, wherein the electrolyte is stable and readily prepared. The electrolyte comprises the platinum salt, dinitrodiamine platinum (Pt(NH3)2(NO2)2), and an alkali metal carbonate or bicarbonate; and the process results in decreased contaminant levels of S, Cl, and P in the electroplated Pt layer, as compared with Pt layers deposited using prior art electrolytes.
It is known that the presence of chromium (Cr) in superalloy coatings (e.g., Cr containing platinum aluminide coatings) increases the corrosion resistance of superalloy components, and hence increases the life of such components, as compared with platinum aluminide coatings which substantially lack Cr.
EP 0821076 A1 to Wing discloses a process for forming a platinum aluminized chromised Ni-based superalloy, wherein the process involves the steps of: 1. forming a chromium enriched surface layer of the superalloy, e.g., by electroplating; 2. heating in a vacuum or protective atmosphere; 3. forming a platinum layer on the chromium enriched superalloy by electroplating, sputtering, etc.; 4. heating in a vacuum or protective atmosphere for one to four hours at 900° to 1150° C.; and 5. aluminizing the chromised, diffused, platinum coated Ni-based superalloy, e.g., by out of pack aluminizing for six hours at 1080° C. Thus, in the process of Wing, Cr and Pt are deposited on the superalloy in separate steps.
U.S. Pat. No. 5,482,578 to Rose et al. discloses a diffusion coating process for the deposition of a coating of chromium-containing PtAl2 on a superalloy substrate. The process of the ′578 patent involves: 1. deposition of a platinum group metal on the superalloy, e.g., by electroplating; 2. heating in vacuo at about 1900° F. for about an hour; 3. diffusion coating the platinum-group metallized superalloy with an Al/Cr powder; and finally 4. heat treating the Al/Cr diffusion coated, metallized superalloy at about 1925° to 2050° F. in hydrogen for about one to two hours. Again, in the process of Rose, et al., Cr and Pt are deposited on the superalloy in separate steps.
As can be seen, there is a need for a process for concurrently depositing platinum and a supplementary constituent on a substrate in a single step. There is a further need for a readily applied Cr containing platinum aluminide coating for superalloy gas turbine engine components. There is a further need for a process for coating superalloy components with a Cr containing platinum aluminide coating, wherein Pt and Cr are deposited on the superalloy surface in a single electrolytic step, such that processing costs are decreased, and productivity is increased. There is also a need for a reliable, stable, effective, and readily available electrolyte composition for co-electrodeposition of Pt and Cr on a substrate.